1. Field of the Invention
The invention relates to a feed or output-coupling device for a coaxial line, in particular a multiple coaxial line.
2. Background of Related Art
A problem which arises in many applications is to connect the inner conductor of a coaxial line to the same potential as the outer conductor, in general to earth. In practice, this can be done, for example, by means of a short-circuited xcex/4 spur line, which is connected in parallel with another line. Thus, in other words, the inner conductor is short-circuited to the outer conductor at the end of the spur line. Although the inner conductor is electrically connected to the outer conductor by connecting the short-circuited xcex/4 spur line in parallel, this parallel circuit does not change the input impedance of the other line if, specifically, the length of the coaxial line is one quarter of the relevant wavelength. Specifically, the short-circuit at the end of the spur line is transformed to an open circuit at the input to the line. This principle is thus dependent on the wavelength and thus to this extent acts only over a narrow bandwidth.
Such circuits may be used, for example, as surge arresters (lightning protection circuits) in coaxial lines in order to connect an inner conductor to the potential of the outer conductor for narrowband applications, that is to say, as a rule, to connect the inner conductor to earth, in the same way as the outer conductor, and thus to earth it.
In fact, a large number of operational situations are knownxe2x80x94which will also be described in the following textxe2x80x94in which the narrowband solution mentioned above is inadequate. Specifically, a large number of operational situations require a broadband solution. As a rule, this necessitates the use of gas-filled surge arresters.
One other important operational case of a broadband application relates to the mobile radio area.
As is known, the majority of the mobile radio area is handled via the GSM 900 network, that is to say in the 900 MHz band. In addition, GSM 1800 Standard has become established, inter alia, in Europe, in which signals can be received and transmitted in an 1800 MHz band.
Multiband antenna devices for transmitting and receiving various frequency band ranges are therefore required for such communication, and these antenna devices normally have dipole structures, that is to say one dipole antenna device for transmitting and receiving the 900 MHz band range, and a further dipole antenna device for transmitting and receiving the 1800 MHz band range.
To this extent, there is also a requirement for omnidirectional antennas with two separately fed antenna systems, in which case each antenna system is intended to be suitable for at least two frequency bands. Depending on the requirement, the individual antenna systems can then be connected or used for one or two frequency band ranges. The implementation of this concept means that, for example, two such multiband antennas are installed alongside one another, although this is linked to the disadvantage that the individual antennas no longer have an omnidirectional polar diagram, since their radiation fields shadow one another. Furthermore, such a concept requires a relatively large amount of space, particularly if at least approximately omnidirectional transmission characteristics are intended to be achieved.
In contrast to the concept mentioned above, it is also in principle known for two corresponding antenna devices to be arranged one above the other. However, this concept can be implemented only if the individual antenna systems are intended for only one frequency band range. The feed and output-coupling apparatus in this case comprises two coaxial cables, which are each routed out of the antenna mast at the level of the relevant antenna device.
A feed or output-coupling apparatus for a coaxial line for a multiband antenna is known, in principle, from DE 23 54 550 A1, with this previously known arrangement comprising an outer conductor and an inner conductor together with a spur line, which is in each case connected to the outer conductor and the inner conductor of a feed line at the side. The outer conductor is short-circuited to the inner conductor at the end of the spur line. The length of the spur line in this case corresponds to one quarter of the wavelength of the waves which pass via the feed line.
In principle, it would be desirable to provide an antenna device which comprises a number of antenna systems arranged one above the other and which should then, in fact, likewise be useable for at least two frequency band ranges. However, multiband or broadband feeding is impossible in this case with known means and solutions.
However, applications which are even simpler than this are also conceivable, in which, for example, a multiple antenna system is operated in only one frequency band range, although no suitable feed or output-coupling apparatus which is easy to handle is known for this purpose.
The object of the invention is thus to provide an improved feed or output-coupling apparatus, in particular for a single-band or multiple frequency band antenna device.
The invention for the first time provides a solution for connecting an inner conductor to the potential of the outer conductor using simple means, to be precise for broadband application, that is to say an application having at least two frequencies or frequency band ranges. In this case, the inner conductor can be connected to earth in the same way as the outer conductor. The concept according to the invention can be extended without any problems to multiple coaxial lines, by means of which it is then possible, according to the invention, to feed multiple, multiband antenna systems, arranged one above the other, without any problems.
The concept according to the invention consists in that two interleaved, short-circuited xcex/4 lines or spur lines are used, for example, for two frequency bands, with the electrical length of the one line being matched to the one frequency, and the electrical length of the other short-circuited line being matched to the other frequency. Since the two short-circuited xcex/4 lines are connected in series, the short-circuits at the feed point result in an open circuit being transformed for each of the two frequency band ranges, as a result of which the outer coaxial line can be fed in a matched manner. The short-circuiting connection in this case results in the inner conductor being connected to the same potential as the outer conductor, so that the inner conductor is connected to earth when the outer conductor is also connected to earth.
For one preferred embodiment, the electrical length of the outer xcex/4 line corresponds to the higher frequency, with the electrical length of the inner coaxial line being matched to the lower frequency. An opposite arrangement is equally possible.
The principle according to the invention can, however, be implemented equally well for even greater bandwidths in that, for example, appropriate matching is also provided for at least one further frequency band range, that is say, for example, a third frequency band range which is offset from the first two frequency bands. In this case, the inner conductor is electrically connected to the outer conductor by three short-circuited xcex/4 lines (spur lines) which are interleaved with respect to one another, with the electrical length of the three short-circuited spur lines being matched to the relevant frequency bands.
One particularly preferred embodiment provides for the inner conductor of the coaxial line to be formed by a further coaxial line, thus resulting, for example, in a triax line. The inner coaxial line may be used, for example, to feed an upper antenna system which covers at least two frequency band ranges, in which case the outer coaxial line can be used to feed a lower antenna system with at least two frequency band ranges in a corresponding manner. The outer conductor of the inner coaxial line is in this case at the same time the inner conductor of the outer coaxial line, which is connected to the same potential by the short-circuited and interleaved xcex/4 lines according to the invention.
If a multiple coaxial line having a number of inner and outer conductors is intended to be fed in a general form, then the principle according to the invention is used in cascaded form, using short-circuiting lines which are interleaved with respect to one another and are provided as a function of the number of frequencies, in order in this way to electrically connect a respective outer conductor to the closest inner conductor on the inside in each stage.
The principle according to the invention makes it possible, without any problems, to feed and to provide output coupling for a number of individual antennas via a common line even, for example, for the case of a multiband, or at least two-band antenna without any problems. This line comprises a multiple coaxial line, for example a triax line when there are two antenna devices arranged one above the other. If the aim is to feed n antennas arranged one above the other, then a coaxial line with n+1 lines is required. In this case, each of the antenna devices arranged one above the other can be used to transmit or receive a number of frequency band ranges, for example two frequency band ranges, three frequency band ranges, etc.
A multiband output-coupling apparatus according to the invention for such multiple coaxial lines allows very good output coupling for the various frequency band ranges to be transmitted, for example for the two bands at 900 MHz and 1800 MHz in the mobile radio area. The good matching which this results in leads to a considerably improved VSWR (voltage standing wave ratio, that is to say to an improved ripple factor or standing-wave ratio).